This section provides background information related to the present disclosure which is not necessarily prior art.
When performing installation or maintenance work on electrically powered equipment, it is necessary to determine the status of an electrical circuit supplying the equipment to ensure it is switched off. Hardware commonly used for isolating plant and machinery may include circuit breakers, isolators, switches, links and fuses. Hardware that has been incorrectly isolated or is faulty can result in electric shock and injury to an operator or damage to the equipment.
Electrical equipment may be incorrectly isolated due to a number of different reasons including faulty contact mechanisms, faulty handles and misaligned handle mechanisms. One method of determining whether the equipment is de-energized is to measure a voltage on the exposed conductors of an electrical circuit supplying the equipment using a voltmeter. This process is widely known as “proving dead” or “testing for dead” and incorporates a three step procedure.
Firstly, an operator needs to ascertain whether the voltmeter is operating correctly by connecting the test leads of the voltmeter to a known energized circuit and verifying that the measured voltage is within defined limits. Secondly, the operator connects the voltmeter to the electrical circuit under test to check the absence of a voltage. Finally, the operator verifies that the voltmeter is working by re-measuring the voltage of the known energized circuit and ensuring that the measured value falls within the limits. This procedure allows the operator to conclude that the voltmeter was operating correctly during the testing of the circuit under test step and that the circuit has been proven de-energized.
U.S. Pat. Nos. 4,804,908, 6,100,679, 6,323,638 describe devices that require a direct connection with the electrical conductors of the circuit under test. A drawback of these devices is that the operator is exposed to hazardous voltages and therefore the test may only be performed by trained and licensed personnel.
U.S. Pat. Nos. 6,531,880, 7,295,130 disclose devices that do not require a direct connection with the electrical conductors of the circuit under test. These devices allow the operator to perform the test by placing the device in the near vicinity of the insulated conductor and measuring the electric and/or magnetic field. A disadvantage of these devices is that they must be placed within a short distance of the circuit under test which may not be possible or at least safe, for circuits enclosed within an equipment cabinet. Furthermore conductive enclosures and barriers attenuate electric and or magnetic fields making the measurements unreliable. In order to perform the test the operator may need to access inside the cabinet, again, requiring the services of trained and licensed personnel.
Other testing devices commonly used are permanently connected to the circuit under test and may take the form of voltmeters or indicating lamps. These devices may incorporate a self testing function in the form of a push button that applies a known test voltage. The operator utilises these permanently connected devices together with the “testing for dead” procedure to prove that the circuit has been de-energized. U.S. Pat. No. 5,986,557 describes a fuse status indicator incorporating pseudo-self testing. A disadvantage of this system however is its complexity. The use of permanently connected testing devices on a high energy power circuit may compromise the ability of the robust power circuit to withstand power system disturbances and their subsequent mechanical forces. Such testing devices may weaken the robustness of the circuit under test. Furthermore such a system is expensive to install.
U.S. Pat. Nos. 6,020,822 and 7,319,315 describe devices that may be used by untrained personnel where the device is plugged into a standard electrical outlet or General Purpose Outlet (GPO). A limiting factor of these devices is that they are unsuitable for checking circuits other than those fitted with a GPO. A further drawback of these devices is that live electrical terminals that potentially convey high energy are accessible to an operator. As such inadvertent contact with the terminals, or damage of the device while in use, may potentially expose the operator to electric burns or electric shock.
A device in the form of a passive test point is also commonly available that conveys the presence of an electric field surrounding an internal circuit to the outside of an electrical enclosure. An operator may then with the assistance of a portable electric field measuring device detect the presence or absence of an internal electric field and hence the presence or absence of voltage on the circuit under test. A drawback of these devices are that in the case of circuits incorporating more than one phase the operator must measure all phases, determine what constitutes an energized or de-energized phase and conclude through processing the results whether the circuit under test is energized or de-energized. A further drawback is that the passive devices do not offer the provision for self testing for the condition when the test point is short circuited to earth and the circuit under test has been isolated.